Cosmic Magic. Hastur is unaffected by an antimagic field spell or other effects that would prevent it from casting spells and any ability check made to identify a spell Hastur casts automatically fails. In addition, when Hastur casts a spell that targets one creature, it can target a total of three creatures with that spell instead.

Eldritch Tentacles. The area within 30 feet of Hastur is covered with writhing, slimy tentacles and counts as difficult terrain. Each creature of Hastur's choice that starts its turn within that area or enters it the first time on a turn must make a DC 20 Dexterity saving throw or take 18 (4d8) bludgeoning damage and be restrained by the tentacles. A creature that starts its turn while already restrained by the tentacles takes 9 (2d8) acid damage and 9 (2d8) cold damage instead. When Hastur moves, creatures restrained by its tentacles move with it.

A restrained creature can use its action to make a DC 20 Strength (Athletics) or Dexterity (Acrobatics) check. On a success, it frees itself.

When a creature drops to  0 hit points while restrained by the tentacles, it dies. Its soul is ripped from its body and travels to Carcosa, while its body melts away and is transformed into a gibbering mouther under Hastur's control. If a creature dies in this way, it cannot be brought back to life by any means except for a wish spell.

Fulvous Dreams. Hastur can cast the dream spell on a creature that has seen a Yellow Sign before and failed its saving throw, even if that creature is on a different plane of existence. 

When it does so, that creature suffers horrible nightmares tinted with a nauseating yellow color and if it fails its saving throw against the spell, it suffers from a madness in addition to the spell's normal effects. While under the effect of this madness, it has disadvantage on Wisdom (Perception) checks and on Constitution saving throws made to keep concentration on a spell, and it is compelled to seek out the next active Yellow Sign, devoting all its actions and ressources to this newfound endeavour.

Once the creature sees the next active Yellow Sign, the madness immediately ends and the sign's effects, if any, are applied.

Immortality. When Hastur is killed, its yellow robes drop to the ground, as if whatever shape supported them suddenly ceased to exist. The robes themselves remain inanimate on the ground, but any humanoid creature that comes into contact with them must make a DC 27 Wisdom saving throw. On a failure, it is overcome by an urge to put these robes on, which requires an action to do. Once it has donned the robes, it immediately ceases to exist, its soul is sent to Carcosa, and in its place, Hastur appears again with all its hit points restored.

If the robes are not donned within the next 24 hours, they fade away, leaving behind a yellow stain. In this case, Hastur cannot manifest a physical body again until the conditions are right or until it is called forth again by its cult.

Incomprehensible. Hastur is utterly incomprehensible and alien to mortals, there exists no such way to use mental effects or magic effectively against it. Hastur automatically succeeds on any Intelligence, Wisdom or Charisma saving throw.
In addition, any creature that attempts to read Hastur's thoughts or attempts to deal psychic damage to Hastur must make a DC 27 Wisdom saving throw or develop a short-term madness.

Innate Spellcasting. Hastur's spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). It can innately cast the following spells, requiring no components:

• at will: dimension door, dissonant whispers, dream, mental prison, symbol, synaptic static, Tasha's mind whip, vortex warp, weird
• 3/day: astral projection, dominate monster, feeblemind, maddening darkness, mass suggestion, mirage arcane, project image, Raulothim's psychic lance
• 1/day: gate, psychic scream, power word kill, time stop, wish

Legendary Resistance (5/day). If Hastur fails a saving throw, it can choose to succeed instead.

Magic Resistance. Hastur has advantage on all saving throws against spells and magical effects.

Reality-Defying Existence. If Hastur is at a point where multiple planes of existence overlap, it exists in all of these planes at the same time, can target creatures in all of these planes and when it uses an ability that creates an area of effect, it affects the same area in all of these planes. This means, if Hastur is on the Material Plane, it can target creatures in the Ethereal Plane, such as those who have cast blink or etherealness, with its spells and attacks, and its Yellow Mist extends into the Ethereal Plane as well.
In addition, Hastur is immune to any spell or effect that would alter its form or send it to another plane of existence.

Multiattack. Hastur can use its Unspeakable Presence. It then casts one of its spells, uses its Yellow Mist or makes three attacks: two with its tattered robe and one with its cosmic orb.

Cosmic Orb. Hastur targets a point within 500 feet of it and hurls an orb of crushing cosmic energy at it. Each creature within 10 feet of that point must make a DC 27 Wisdom saving throw. On a failure, it takes 18 (4d8) force damage and 18 (4d8) psychic damage and is knocked prone, on a success it takes half as much damage without any other effect.

Tattered Robe. Melee Spell Attack: +19 to hit, reach 30 ft. one target. Hit: 19 (2d8+10) slashing damage plus 27 (6d8) psychic damage and the target must make a DC 27 Constitution saving throw or suffer an otherworldly wound which glows with faint yellow light and causes searing pain.
A creature wounded in this way has its movement speed halved and has disadvantage on attack rolls and Constitution saving throws made to maintain concentration.
The wound lasts until it is cured by greater restoration or similar magic.

Unspeakable Appearance. Any creature of Hastur's choice that is within 120 feet of Hastur and is aware of it must make a DC 27 Wisdom saving throw or develop a short-term madness. On a success it is immune to Hastur's Unspeakable Appearance for the next minute.

Yellow Mist (Recharge 5–6). The area within 120 feet of Hastur fills with cosmic fog of nauseating yellow color, causing it to be lightly obscured. The fog is filled with incomprehensible, terrible whispers and visions from Carcosa and lingers for one minute or until it is dispersed by strong winds. As the cosmic fog spreads out, each spell in the area ends and spells of 5th level or lower cannot be cast. Hastur itself and its spells are unaffected by the Yellow Mist.

In addition, any creature that starts its turn within the fog or enters it for the first time on a turn must make a DC 27 Wisdom saving throw. On a failure, it is charmed by Hastur and cannot willingly move more than 120 feet away from it. On a success, it takes 45 (12d8) psychic damage instead and suffers no other effects.

Yellow Sign (1/day). Hastur touches a solid surface and inscribes the Yellow Sign upon it. Once inscribed, the sign remains for one year, but only becomes active during certain nights when the baleful light of Carcosa shines in the night sky as a yellow star.      

While the Yellow Sign is inactive, it is invisible, but a glimpse of its magic can be detected with a detect magic spell or similar ability. If a creature senses the Yellow Sign's magic in this way, it must make a DC 27 Wisdom saving throw or develop a long-term madness.

When the Yellow Sign becomes active, it glows with yellow light. Any creature that can see the Yellow Sign must make a DC 27 Wisdom saving throw or be charmed by Hastur for 24 hours. While charmed in this way, the creature obeys Hastur's will as per the dominate monster spell. Once the effect ends on a creature, it develops a long-term madness and becomes an eligible target for Hastur's Fulvous Dreams. If a charmed creature dies, Hastur can manifest physicially in the location of its corpse.

In addition, the Yellow Sign can bear one of the effects described in the symbol spell. The Yellow Sign is considered a 10th level spell for the purpose of removing it with dispel magic or similar abilities.

Reveal Visage. As a bonus action, Hastur reveals the true shape beneath its robes to one creature that can see it and is within 5 feet of him. That creature must make a DC 27 Charisma saving throw or be frightened for the next minute. While frightened in that way, it is paralyzed and its Wisdom score is reduced by 1d4 at the start of each of its turns, as the revelation is too incomprehensible, horrific and alien for its mortal mind.     
The creature dies if this reduces its Wisdom to 0. Otherwise, the reduction lasts until cured by greater restoration or similar magic.

A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. Once the effect ends for it, the creature develops an indefinite madness. In addition, if a creature's saving throw is successful or the effect ends for it, it is immune to the effects of Reveal Visage for the next minute.

Otherworldly Retribution. When a creature deals damage to Hastur, otherworldly yellow light seeps through the wound. Hastur can use its reaction to force that creature to make a DC 27 Wisdom saving throw. On a failure, it must use its reaction to move into a random direction as far as its speed allows and until the end of its next turn, it has vulnerability to psychic damage.

Hastur can take 5 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Hastur regains spent legendary actions at the start of its turn.

Attack. Hastur makes an attack with its cosmic orb or tattered robe.

There is no Escape. Hastur makes a Wisdom (Perception) check to search for a creature that attempted to take the hide action on its last turn and moves up to half its speed. If the check is successful, the creature takes 18 (4d8) psychic damage and must make a DC 27 Wisdom saving throw or is frightened until the end of its next turn as it can feel Hastur's otherworldly gaze.

Tentacle Lash. Hastur lashes out with its Eldritch Tentacles, targeting a creature within 60 feet of it. The target must make a DC 27 Dexterity saving throw. On a failure, it takes 18 (4d8) bludgeoning damage, is  restrained by the tentacle and pulled up to 30 feet straight towards Hastur.

Cast a Spell (Costs 2 Actions). Hastur casts one of its at-will spells.

Whispers of the Stars (Costs 2 Actions). All creatures of Hastur's choice that are charmed by it start hearing eerie, unsettling whispers from distant worlds, as if the stars were talking to them - or is it Hastur itself who is speaking? Each of these creatures must use its reaction to move up to its speed and to make one weapon attack against a target of Hastur's choice. If this attack hits, it deals an additional 18 (4d8) psychic damage.

Devour (Costs 3 Actions). Hastur devours the essence of all creatures restrained by its Eldritch Tentacles. Each of these creatures must make a DC 27 Charisma saving throw. On a failure, a creature takes 54 (12d8) necrotic damage and gains a level of exhaustion, on a success, it takes half as much damage and is not exhausted. Hastur regains hit points equal to the total necrotic damage dealt.